The invention relates to an add and drop node for an optical fiber network using wavelength division multiplexing (WDM).
Optical multi-channel systems employing wavelength multiplexing are used both in new networks and in order to enhance the transmission capability of existing optical fiber networks. Thus, information channels which previously had to be transmitted on a plurality of separate fiber pairs are forwarded on a single fiber pair in WDM networks. Using optical wavelength division multiplexed channels means that a plurality of serial information signals, i.e. a plurality of serial binary signals, are transmitted on the same optical fiber by modulating such a serial signal on a light signal, also called carrier, having a definite wavelength and then combining the modulated light signals in an optical coupler or optical multiplexer to a composite light signal on the considered optical fiber. The signal primarily modulated on a monochromatic light signal or carrier together with the carrier can be called a channel or traffic channel.
Self-healing optical fiber networks having a ring configuration are disclosed in U.S. Pat. No. 5,442,623, but they are not particularly adapted to WDM-signalling. A similar network designed for WDM-traffic is disclosed in the International patent application PCT/SE98100136. The networks described in these documents use an extra protecting fiber pair between each pair of nodes.
Optical wavelength multiplexing can generally be used in different optical fiber network configurations or architectures having e.g. only a single fiber pair between a pair of nodes. Such an architecture is the FlexBus(trademark) concept as described in B. S. Johansson et al., xe2x80x9cFlexible bus: A self-restoring optical ADM ring architecturexe2x80x9d, Electronics Letters, 5th Dec. 1996, Vol. 32, No. 25, and U.S. patent application 08/421,734, this architecture comprising a ring configuration of optical links connecting a plurality of nodes. The FlexBus(trademark) concept has emanated from the need for protecting ring networks against fiber cuts and optical amplifier failures, and to solve the problem, often associated with ring network architectures, of circulating signals and noise. In the FlexBus(trademark) architecture one section of the fiber ring is always made passive or inactive by means of optical switches or amplifiers. This intentionally introduced break effectively eliminates all problems associated with circulating signals and hence allows that less circuit components can be used and circuit elements having lower performance can be used, while still retaining the shortest longest path possible. In the case of a real failure of a link, that link which previously has been intentionally made inactive is made active and the failed link now becomes the inactive link, what can be described by having the inactive link moved from its former position to the failed section. This procedure is called that xe2x80x9cthe bus flexesxe2x80x9d, and thereby the traffic is restored.
In the FlexBus(trademark) channel blocking or selection filters placed in the lines are not needed, which alleviates the problems associated with concatenated filtering. The signal from one transmitter can be sent in both directions simultaneously without causing interference, and the same wavelength can be used in both directions, thus allowing the same number of bidirectional connections to be set up as the number of wavelengths that are used in the network.
With the maturing of filtering and switching technology it would, however, be beneficial to be able to reuse wavelengths more than once in order to be able to set up more connections and thus increase network capacity for the limited number of wave-lengths that are feasible in a network with regard to available optical amplifier gain-bandwidth, realistic filter bandwidths and frequency stability of filters and light sources. Thus another implementation of a node architecture, based on the FlexBus(trademark) but including a plurality of blocking filters and switches connected in-line, i.e. in the direct path of a fiber of the network or bus through the node, was invented and is disclosed in the published International patent application WO 96/31025 and is called the xe2x80x9cRearrangeable FlexBus(trademark)xe2x80x9d. That implementation is capable of a very efficient use of the wavelengths. In the published International patent application WO 96/24998 an algorithm scheme for wavelength allocation in Rearrangeable FlexBus(trademark) networks is disclosed.
The general architecture of the Rearrangeable FlexBus(trademark), see also the published International patent application WO 96/24998 and the U.S. patent application xe2x80x9cAn optical WDM network having an efficient use of wavelengths and a node thereforxe2x80x9d, filed at the same time as the present application, is very efficient in using the available wavelengths and also very flexible as to the number of connections which can be set up in the network. Partly depending on the actual way in which networks often have been implemented, partly because of the requirement for truly adjacent traffic in networks, it appears to often exist an interest of finding good network solutions for adjacent traffic demands, i.e. for traffic between neighboring stations or nodes. For a network having only such a kind of traffic, the channel by channel configurability is not necessarily required, and it is thus of interest to search for a simpler design of an add and drop node for this type of traffic pattern compared to that disclosed in the above-cited, simultaneously filed U.S. patent application.
It is an object of the invention to provide an add and drop node for a network of the kind Rearrangeable FlexBus(trademark) as described above and for traffic only between neighboring nodes, the node having a minimum of in-line components and enhancing the reliability of the network but still having the good properties of the FlexBus(trademark) and allowing an efficient use of wavelengths.
The problem to be solved by the invention is how to achieve, for a network of the kind Rearrangeable FlexBus(trademark) having traffic between only neighboring nodes, a node construction operating substantially as the nodes of that bus allowing an efficient wavelength allocation in the network and allowing the network and the nodes to operate in a reliable way.
Thus generally, an add/drop node is designed to be connected in an optical fiber WDM network and for communication only with neighbouring nodes. The network has a ring configuration of two fibers carrying light signals in opposite directions and comprises links connecting neighbouring nodes. For protection purposes, the network always has one link which is inactive and carries no light signals and the network has switching facilities allowing the inactive link to be made active and another link to be made the inactive link. The network carries light signals containing useful information in a plurality of separate wavelength bands. In the conventional way the node has drop couplers and add couplers for each direction for taking out a share of light signals at the node and adding light signals in the node respectively and receivers for receiving light signals in wavelength bands in the node and transmitters for transmitting light signals in wavelength bands from the node into the network. A band blocking filter is connected between a drop coupler and an add coupler which are used for light signals in one direction for blocking in the same direction all wavelengths which are received in the node from that direction. A switch is connected to a receiver and the drop couplers for allowing the receiver to receive from either one of the two opposite directions and/or a switch is connected to a transmitter and the add couplers for allowing the transmitter to transmit in either one of the opposite directions. Preferably, two separate switches are provided, one connected for receiving and one for transmitting.
The receivers can be divided in two groups of west port receivers and east port receivers. One switch is then preferably connected to all the receivers of one group for allowing all the receivers of this group simultaneously receive from either one of the opposite directions. Then an optical demultiplexer can be connected between the switch and the group of receivers.
The transmitters can also be divided in two groups of west port transmitters and east port transmitters. One switch can then be connected to all the transmitters of one group for allowing all the transmitters of this group simultaneously transmit in either one of the opposite directions. An optical multiplexer is then advantageously connected between the switch and the group of transmitters.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.